A boring work of forming a bore in a cylinder block of an engine is carried out in a process of manufacturing a vehicle. In the boring work, a tool for boring is made to advance to and retreat from a cylinder block to form a bore while the cutting tool is rotated, and thus a spiral machining mark (hereinafter referred to as “cross hatch”) occurs on the inner surface of the bore. This cross hatch is used as a passage for engine oil (oil pit). When the surface roughness and surface property of the inner surface of the bore are deteriorated by the cross hatch, the sliding resistance of a piston sliding through the bore increases, so that it is impossible to make an engine have a desired performance. Therefore, after a boring is formed by the boring work, a honing work is generally executed to execute polish-finishing on the inner surface of the bore to the extent that an oil pit remains. After the honing work, a surface inspection to detect polishing residues on the inner surface of the bore is executed.
As a technique of inspecting the surface of a bore is known a method of imaging the inner surface of the bore to generate a digital image, binarizing the brightness values of the digital image on the basis of a predetermined threshold value and extracting lines regarded as polishing residues. Furthermore, in the extracting step of the lines, the mesh angle of the cross hatch is predetermined, and thus only lines extending along the angle concerned are set as extraction targets. Therefore, lines which are regarded as polishing residues of the machining mark are efficiently extracted (for example, see Patent Document 1).